(III) In-house recycled items
(Recycled resources): in-house recycled steel scrap
O2 gas was used for control of carbon composition in steel. Electric power was used mainly for steeling and forging processes and LNG was for preheating of furnaces and heat treatment. Steel scrap and molding sand disposed outside the factory were taken into account as solid wastes.
In order to add CO2 emission to the waste items, it was assumed that CO2 emission is derived from combustion gas of LNG and from all the 02 gas which reacted with carbon. CO2 emitted from burned wooden mold frames were excluded.
According to the main manufacturing process flow in Figure 3, inventory data in manufacturing cast/forged steel parts were calculated by using the matrix-method with 10 x 10 process matrices. Column vectors were selected as shown in Table 3.
Table 3 Inventories/processes as column vectors in LCI analysis of cast/forged steel parts
Column vectors of energies and materials were given as quoted inventory data. Those of manufacturing processes were given as process data allocated on the basis of the factory data. Since a considerable amount of steel scrap is recycled in-house, two imaginary column vectors were set for dealing with the loop-type process flow in Figure 3 [2].
After process quantity matrices which mean correlation of the items in Table 3 were calculated, quantities of environmental load items were summed up. Iron ore, coal, crude oil, natural gas, steel scrap, CO2 emission and solid wastes were considered as environmental load items.
Of the results of estimate, CO2 emission profiles of cast/forged steel parts are shown in Figure 4 and Figure 5. CO2 emission of each process includes the CO2 emitted in the production stage of energies. CO2 emissions per unit of cast/forged steel parts are approximately 0.92 (kg/kg product) and 3.8 (kg/kg product) respectively. Nearly half of the difference between them are attributable to the LNG combustion in the heat treatment process of forged steel parts.
3.3 Remarks on estimation
According to published inventory data [3], the CO2 emissions in manufacturing cast/forged steels are 2.26 (kg/kg product) and 3.42 (kg/kg product) respectively. Since there are some difference between the authors' results and published inventory data, it should be made several remarks on the estimation.
Firstly, the weight ratio of steel scrap as main material to the steeling weight in the factory accounts for nearly 85 (%). Since steel scrap isn't counted as inventory data, the sensitivity of the authors' result depends much on its percentage.
Next, there is considerable difference between in-house recycled steel scraps discharged from a unit of cast/forged steel parts. That is, in the steeling process of them, approximate same amounts of in-house recycled steel scrap are used.
